add a better variable override mechanism

The override mechanism proposed by @stanislav-zaprudskiy in #352 is
problematic because --warn-undefined-variables makes noise about it:

```shellSession
$ make
Makefile:67: warning: undefined variable 'GO_BUILDFLAGS'
Makefile:68: warning: undefined variable 'GO_LDFLAGS'
Makefile:69: warning: undefined variable 'GO_TESTFLAGS'
Makefile:70: warning: undefined variable 'GO_TESTENV'
Makefile:71: warning: undefined variable 'GO_BUILDENV'
env  go build -mod vendor  -ldflags '-s -w ...
```

However, through much experimentation, I discovered that the += operator
behaves in the intended way, as documented in the new generated comment.

Author: majewsky

Makefile

@@ -50,10 +50,12 @@ install-reuse: FORCE
 
 prepare-static-check: FORCE install-golangci-lint install-modernize install-shellcheck install-go-licence-detector install-addlicense install-reuse
 
-GO_BUILDFLAGS = -mod vendor
-GO_LDFLAGS =
-GO_TESTENV =
-GO_BUILDENV =
+# To add additional flags or values, specify the variable in the environment, e.g. `GO_BUILDFLAGS='-tags experimental' make`.
+# To override the default flags or values, specify the variable on the command line, e.g. `make GO_BUILDFLAGS='-tags experimental'`.
+GO_BUILDFLAGS += -mod vendor
+GO_LDFLAGS +=
+GO_TESTENV +=
+GO_BUILDENV +=
 
 # These definitions are overridable, e.g. to provide fixed version/commit values when
 # no .git directory is present or to provide a fixed build date for reproducibility.

internal/makefile/makefile.go

@@ -211,10 +211,12 @@ endif
 	}
 
 	if isGolang {
-		build.addDefinition("GO_BUILDFLAGS =%s", cfg.Variable("GO_BUILDFLAGS", defaultBuildFlags))
-		build.addDefinition("GO_LDFLAGS =%s", cfg.Variable("GO_LDFLAGS", strings.TrimSpace(defaultLdFlags)))
-		build.addDefinition("GO_TESTENV =%s", cfg.Variable("GO_TESTENV", ""))
-		build.addDefinition("GO_BUILDENV =%s", cfg.Variable("GO_BUILDENV", ""))
+		build.addDefinition("# To add additional flags or values, specify the variable in the environment, e.g. `GO_BUILDFLAGS='-tags experimental' make`.")
+		build.addDefinition("# To override the default flags or values, specify the variable on the command line, e.g. `make GO_BUILDFLAGS='-tags experimental'`.")
+		build.addDefinition("GO_BUILDFLAGS +=%s", cfg.Variable("GO_BUILDFLAGS", defaultBuildFlags))
+		build.addDefinition("GO_LDFLAGS +=%s", cfg.Variable("GO_LDFLAGS", strings.TrimSpace(defaultLdFlags)))
+		build.addDefinition("GO_TESTENV +=%s", cfg.Variable("GO_TESTENV", ""))
+		build.addDefinition("GO_BUILDENV +=%s", cfg.Variable("GO_BUILDENV", ""))
 	}
 	if sr.HasBinInfo {
 		build.addDefinition("")